1. Field of the Invention
The present invention is directed to water based ink compositions suitable for use in thermal ink-jet ink printers. Specifically, the water based ink compositions described in the present invention contain a specific humectant or a mixture of specific humectants. The use of such humectants provide for an ink composition having good print quality in thermal ink-jet ink printers, good drying time and reduced kogation on the resistor surface of the print head of thermal ink-jet ink printers.
2. State of the Art
Aqueous ink-jet ink compositions have been commercially employed in thermal ink-jet printing. Such formulations generally comprise water and a water compatible colorant. The print head of such printers generally comprise a resistor and an orifice plate and, during operation, a drop of ink is formed on the resistor surface. Heating of this drop by the resistor leads to explosive bubble formation which forces the drop of ink from the resistor, through the orifice plate in the print head and onto the substrate.
One of the serious problems encountered in employing aqueous ink-jet ink compositions in thermal ink-jet printing is the difficulty in maintaining the reliability of the printer (i.e., the ability of the printer to provide quality print with minimal shutdown) without sacrificing print quality. Specifically, one concern arises from crusting of the ink composition at the orifice plate of the printer. This concern is compounded by the fact that during operation, the resistor is generally heated to an elevated temperature (e.g., about 300.degree. to about 400.degree. C.) and by the fact that the ink has to pass through very small orifices (about 40 to 70 microns in diameter) in the orifice plate in order to reach the substrate. Accordingly, when used in thermal ink-jet printers, aqueous ink-jet ink compositions are prone to evaporation. Such evaporation leads to unacceptable crusting and clogging of the orifices because the viscosity of the ink composition increases substantially after some of the volatile components have evaporated.
Crusting and clogging of these orifices is unacceptable because it reduces print quality by reducing drop volume delivered to the substrate and ultimately leads to failure of the printer to print which reduces the reliability of the printer.
In order to overcome this problem, it is common to employ a humectant in the aqueous ink-jet ink compositions. The humectant improves the ink composition by reducing the rate of evaporation and accordingly, reduces the crusting and clogging of print head orifices. Suitable humectants heretofore employed in ink compositions include ethylene glycol, propylene glycol and the like.
However, the use of such humectants still leads to reduced print quality in thermal ink-jet printers in spite of the fact that these humectants provide reduced rates of evaporation. Specifically, the addition of such prior art humectants to aqueous ink compositions used in thermal ink-jet printers results in their thermal decomposition on the surface of the resistor. This decomposition leads to residue deposition on the resistor's surface which process is referred to in the art as "kogation". In turn, such deposits act to insulate the thermal heating of ink drops on the resistor surface leading to reduced bubble formation, decreased ejection velocity of the ink drops, and reduced drop volume delivered to the substrate, all of which result in reduced print quality and can eventually lead to bubble formation failure resulting in failure of the thermal ink-jet printer to print.
In view of the above, ink compositions employing humectants which reduce the rate of evaporation of the composition thereby reducing the rate of crusting and clogging of the ink composition at the orifice plates of thermal ink jet printers and which provide reduced kogation problems would be particularly desirable.